1. Field of the Invention
The present invention relates to a frequency sweep circuit and, more particularly to, a frequency sweep circuit used in a multisymbol modulator/demodulator for transmitting a digital signal.
2. Description of the Prior Art
FIG. 1 shows an example of a conventional carrier wave recovery system used in digital radio transmission.
Referring to FIG. 1, a modulated wave input from a signal input terminal IN is multiplied by a multiplier 1 with a recovered carrier wave output from a voltage-controlled oscillator 6. An unnecessary higher harmonic is removed by a low-pass filter 2 from an output signal from the multiplier 1, and the resultant signal serves as a demodulated baseband signal. Upon sampling and quantization, this signal is converted into a digital signal by an analog-to-digital converter 3. From this digital signal, a phase difference detector 4 detects a phase difference between the carrier wave and the recovered carrier wave, and outputs a control signal in accordance with the difference. The control signal is smoothed by a loop filter 5 serving as a low-pass filter and serves as a carrier wave phase control signal for the voltage-controlled oscillator 6. An analog sweep unit 7 connected in an AC manner to the control signal is an analog oscillator using part of a positive feedback loop as an input/output terminal. The analog sweep unit 7 widens the phase locking range by oscillating a sweep signal when phase locking is stepped out on the basis of the state of the control signal, or it stops the oscillation in the phase-locked state.
In the conventional carrier wave recovery circuit described above, however, since the analog oscillator is used in the main part of the frequency sweep circuit, this circuit is difficult to be mounted in an IC, and the oscillation condition easily changes. Further, the conventional circuit has problems such as the limitation of the oscillation frequency and the amplitude.